This invention relates to a novel method and materials for producing digitally imaged short run gravure printing plates and cylinders.
There are four principal traditional printing processes; flexography, offset lithography, screen printing and gravure. Each of these processes is distinguished by the location of the ink in relationship to the surface of the master and which areas of the master provide the non-ink or background areas. Flexographic plates have a raised surface that accepts the ink, the background being the recessed surface. In offset lithography, the ink and the background are coplanar and are differentiated chemically. Screen printing prints ink through holes in the master, with the background being provided by the remaining master surface. Gravure has the ink residing in indented cells, with the background being provided by the remaining upper surface.
Each printing method demands its own types of ink, its own imaging system and its own presses. Each process has its own advantages and disadvantages.
Despite the fact that gravure is regarded as a very simple process compared to other printing processes, its use is limited in commercial printing. xe2x80x9cGravure Process and Technologyxe2x80x9d from the Gravure Association of American (page 380) outlines the advantages and disadvantages of gravure. Gravure is more adaptable to less expensive papers and gives better image quality and color consistency than the other methods. However, the process of engraving gravure cylinders is costly and requires a significant amount of time. This makes the gravure process inappropriate for short runs and, indeed, it finds its place in very long runs of up to and beyond a million impressions.
Gravure cylinders are traditionally prepared either by imaging a photoresist through a film and them chemically etching the metallic surface of the cylinder or by directly engraving the cylinder with some type of engraving tool. A new method of preparation of gravure plates by precise digital indentation of cells is described in Israeli Patent No. 126,883 owned by the owners of the present invention. In recent years, with the advent of computers, origination for reproduction by printing processes has become available in digital form. Much work has been done in imaging printing plates digitally and, more specifically, using a modulated laser beam. An example of such an imaging system is the inventor""s application PCT/IL97/00028, xe2x80x9cAn Imaging Apparatus For Exposing A Printing Member And Printing Members Thereofxe2x80x9d. This patent application is concerned with digital offset lithographic printing using a laser imaging system.
Because of the necessity for engraving specific holes to produce the cells needed for gravure, there have been many attempts to use lasers for digital imaging. For example, U.S. Pat. No. 3,636,251 to Daly et al. describes a system for engraving intalgio printing plates by forming cells in a metal plate using a pulsed output laser. UK Patent Application, GB 2034636A to Pugsley claims that the Daly patent method has the disadvantage that it tends to produce rims around the gravure cells. The Pugsley patent claims an advantage in using polymeric printing blanks for laser engraving, where such blanks have high thermal conductivity. The areas struck by the laser are vaporised. Carbon black may be incorporated into the polymer to improve absorption of the laser energy. More recently, U.S. Pat. No. 5,126,531 to Majima et al. described a method of producing a gravure printing plate using a thermoplastic resin sheet containing about 20 percent carbon. The plate was wrapped around a cylinder and imaged by a semi-conductor laser beam.
Laser imaging as described in the above patents requires large amounts of energy to ablate cells of the required depth for gravure printing. This is achieved either by expensive high-powered lasers or by long imaging times.
It is necessary for gravure plates to be made of materials which can withstand the high pressures involved in gravure printing. Both the printing press and the cleaning blade exert strong pressure on the gravure plate. This is not so for flexography, which is a relatively low-pressure process. Therefore, methods devised for producing flexographic plates, such as the method described in U.S. Pat. No. 5,607,814 are unsuitable for producing gravure plates.
It would be desirable to find a simpler and faster method of producing gravure cylinders or plates that would not require expensive lasers, so that the quality of gravure could be enjoyed by print purchasers who only require some thousands of impressions per job.
Accordingly, it is a broad object of the present invention to overcome the problems of the prior art and provide a novel gravure short-run plate using an integral UV mask.
In accordance with a preferred embodiment of the present invention, there is provided a method for preparing a gravure printing blank usable in a gravure printing process, said method comprising the steps of:
providing an image-ready printing blank comprising:
a substrate;
a UV curing layer; and
an IR absorbing, UV opaque layer;
digitally imaging said printing blank with a digital laser imaging system using an IR laser, such that the IR irradiation causes selective ablation of areas of said IR absorbing layer, forming an image structure having exposed and unexposed areas of said UV curing layer;
flood-curing said printing blank with UV radiation to cure said exposed areas of said UV curing layer in said image structure; and
washing said printing blank so that remaining IR absorbing layer areas and said unexposed image structure areas are removed,
such that the remaining UV cured image structure areas of the UV curing layer form cells for holding printing ink for use in the gravure printing process.
In accordance with another aspect of the invention there is provided an image-ready gravure printing blank, comprising:
a substrate;
a UV curing layer disposed on said substrate; and
an IR absorbing, UV opaque layer.
In accordance with yet another aspect of the invention there is provided a gravure printing blank usable in a gravure printing process, said printing blank comprising:
a printing blank comprised of a substrate, a UV curing layer and an IR absorbing, UV opaque layer;
said printing blank having been imaged with a digital laser imaging system using an IR laser, such that the IR irradiation causes selective ablation of areas of said IR absorbing layer forming an image structure having exposed and unexposed areas of said UV curing layer;
said printing blank having been further cured with UV radiation to cure said areas of said UV curing layer which have been exposed by said image structure; and
said printing blank having been further washed so that areas which have not been UV cured are removed,
such that the remaining UV cured areas of the UV curing layer form cells for holding printing ink for use in the gravure printing process.
In a preferred embodiment, a gravure printing blank is provided that can be easily and quickly imaged by means of digital laser imaging. The polymer-metal printing blank is provided as polymeric layers coated onto metal, wherein the polymer surface interfaces with the printing ink and the metal provides the high dimensional stability. The exposed polymer coat is an infrared (IR) absorbing layer. Selective irradiation with IR causes ablation of those areas of the IR absorbing layer which are exposed to IR. After IR irradiation has selectively ablated the exposed surface, the plate is flooded with UV radiation. Below the IR absorbing layer is an ultraviolet (UV) curing layer. Those areas in which the IR absorbing layer has been ablated allow passage of the UV light to the UV curing layer. The areas in which the IR absorbing layer remains are not cured. Subsequently, the remaining IR absorbing layer and the uncured areas of the UV curing layer are washed away leaving the hardened background areas of the UV curing layer on the gravure plate or cylinder, forming the cells.
In a further alternative embodiment of the invention, the gravure printing plate is provided in a two-part construction with an additional infrared sensitive film provided with the IR absorbing layer on it. The UV curing layer is provided on the substrate. For use in the printing process the two parts are placed together with the film resting on the UV curing layer. The image is made on the IR absorbing layer by processless ablation, then the IR absorbing layer is used as a mask during UV flood curing. After the curing stage the film is peeled away along with the IR absorbing layer. The remaining UV curing layer is then washed to remove the uncured portions, forming cells for holding the ink during the gravure printing process.
In yet a further alternative embodiment of the invention, the gravure printing plate is provided in a two-part construction with an additional infrared sensitive film provided with the IR absorbing layer on its underside. The two parts are placed together with the IR absorbing layer resting on the UV curing layer. The image is made on the IR absorbing layer through ablation transfer, then the transferred layer acts as a mask during UV flood curing. The remaining UV curing layer is then washed to remove the uncured portions, forming cells for holding the ink during the gravure printing process.
In any of the embodiments, an optional second UV flood-curing may be performed to complete curing to give a very insoluble film capable of resisting the solvents of gravure inks such as toluene and ethyl acetate.
In any of the embodiments, an optional baking treatment at 200xc2x0 C.-250xc2x0 C. for a period between 1 and 10 minutes may also be performed for further improvement of solvent resistance and adhesion.
In yet another alternative embodiment, after the cells have been formed, in any of the embodiments, the metallic substrate of the plate may be etched to deepen the cells. In this case, it is possible to remove the UV curing polymer layer completely and use the cells etched in the metallic substrate for the printing plate.
In yet another further embodiment, after the optional etching process, the metallic substrate may go through a plating process. This allows a longer run using the gravure plate.
Other features and advantages of the invention will become apparent from the following drawings and descriptions.